Technical Field
The present invention relates to a methane reforming reactor used to convert methane gas to hydrogen through the use of a tube and shell heat exchanger in which the heat exchanger contains a tube surrounded by a shell. The shell forms an annulus around the tube that allows for convective heat exchange between the tube and a heating medium flowing in the annulus and also maintains high temperatures during a reaction. The heating medium flows into the annulus through a plurality of hot air inlet ports positioned along the shell of the reforming reactor.
Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
Hydrogen, as a fuel has gained much popularity in the present world energy sector due to its potential advantages over hydrocarbon fuels for its clean combustion characteristics and higher calorific value. Hydrogen is produced by a number of ways such as electrolysis, steam methane reforming, auto thermal reforming, partial oxidation reforming and some extensions of these processes. Hydrogen production via electrolysis is an expensive method due to the high production costs involved because of electricity production. Other processes use hydrocarbons as the main reactant for hydrogen production. Among all of the methods above, steam methane reforming (SMR) process is the cheapest, oldest and most widely used method to produce hydrogen commercially worldwide. Steam reforming is, in industrial practice, mainly carried out in reactors referred to as steam reformers, which are essentially fired heaters with catalyst filled tubes placed in the heater. The inlet feed that is methane and steam (along with some traces of hydrogen), enter from one end of the tube and leave as syngas from the other end after the endothermic steam methane reforming reaction takes place.
The above mentioned process may also be carried out in reactors referred to as heat exchange reformers wherein the heat required for the reaction is supplied predominantly by convective heat exchange. The tubes are essentially filled with catalyst. The heat required for the reaction in a convectively heated reformer is supplied by flue gas or process gas or by any available hot gas. The heat and mass balance is considered only on the process side (tube side), thus depicting no difference between heat exchange reforming and fired tubular reforming. The process schemes differ only in the amount of latent heat in the flue gas or process gas and the way in which this heat is used.
These heat exchange reformers are usually installed in combination with another fired tubular reformer placed inline. In this case, the former one is termed as a pre-reformer and the latter one as reformer. To avoid the use of the fired tubular reformer downstream of the pre-reformer, the reforming in the pre-reformer or the heat exchange reformer should be enhanced in order to give higher conversion of methane into hydrogen. Since steam methane reforming is a highly endothermic reaction, the heating medium which enters the heat exchange reformer at one end and leaving from the other end is not sufficient enough to give high conversions of methane to hydrogen.
Conventional shell and tube reforming reactors have one inlet for the heating source in the shell side of the reformer. A reforming reactor of the shell-and-tube configuration can have a shell-side fluid flow path around a tube bundle with a longitudinal configuration. The shell-side around a fluid inlet may be equipped with a distributor plate below the lower end of the tube bundle, and a flow sleeve in an enlarged-diameter discharge annulus at an upper end adjacent to the tube sheet to prevent short-circuiting of the shell-side fluid into the shell-side fluid outlet. Such elongated shells have low and high temperature ends where the fluid inlet to the shell side is at a high temperature for receiving hot gas feed and the tube side inlet adjacent to the low temperature end for receiving a reactant gas feed. The tube bundle may have an inlet secured to the tube sheet for receiving a feed mixture and for discharging product gases adjacent to the shell side to be mixed with the hot gas stream. Another conventional design for the reactor includes a slight alteration on the tube side in which a plurality of ring baffles and lattice baffles provide a lower shell-side pressure drop. Longitudinally spaced traverse ring baffles with one or more longitudinally placed space guides can be positioned along the tube bundle.
A shift reactor can be placed intermediate to two conventional reforming reactors of the shell and tube type to convert carbon monoxide in the outlet of the first reactor to carbon dioxide. The shell and tube configuration is typical to the conventional ones discussed earlier however with a modification of introducing an intermediate shift reactor for carbon monoxide conversion. The shell sides of both the reactors include an inlet for a hot gas feed whereas the tube side has an inlet for the reactant gases. One design includes a disposed reformer tube with partially filled catalyst within a radiant section. The reformer may also include a transition section coupled to the radiant section, a convective section coupled to the transition section, and a plurality of pre-reformer tubes disposed in the transition section. The plurality of pre-reformer tubes can be filled with a second catalyst and fluidly coupled to the reformer tube via a line external to the plurality of pre-reformer tubes. The plurality of pre-reformer tubes include at least one extended surface disposed. The second reformer can be coupled to the reformer tube and to an oxidant source. Another steam/hydrocarbon reformer employing a convection-heated pre-reformer is based on a design in which the pre-reformer contained catalyst-filled tubes are disposed in a transition section between radiant and convection sections. The pre-reformer tubes are transverse to the flow of flue gas from the radiant section. The process of firing the radiant section to produce hot flue gases allow the gases to pass through the transition and convection sections of the reformer. The flue gases then pass over the hydrocarbon feed stream through a preheat exchanger for heat exchange with the hydrocarbon feed.
The above-mentioned conventional designs for shell and tube configuration reformers focus on heating the reformate feed within several zones of the reactor or within several reactors in a manner that is not quite cost effective. Also, tube bundles containing baffles and supports increase the cost for the material installed in the reformer. Moreover, temperature profile is not given its due importance in the design of the reforming reactor.
In order to address these disadvantages a reforming reactor is disclosed which provides superior performance and has advantages including: (1) capability to maintain high temperature in the shell side annulus of the reactor which in turn enhances the conversion of methane, and (2) providing greater conversion when compared to a reactor of the same length with only one inlet at the same mass flow rates of heated air in the shell.